1. Field of the Invention
The present invention relates to a method and apparatus for optimizing dispersion in an optical fiber transmission line in accordance with the power level of an optical signal travelling through the optical fiber transmission line. More specifically, the present invention relates to a method and apparatus which sets the dispersion at a specific point along the optical fiber transmission line to zero, and then adds dispersion downstream of the specific point to optimize the total dispersion in accordance with the power level of the optical signal.
2. Description of the Related Art
Optical transmission systems using fiber optical transmission lines are being used to transmit relatively large amounts of information. For example, optical transmission systems at 10 Gb/s are now in practical implementation in trunk-line optical communications. However, as users require larger amounts of information to be rapidly transmitted, a further increase in the capacity of optical transmission systems is required.
Time-division multiplexing (TDM) (including optical time-division multiplexing (OTDM)) and wavelength-division multiplexing (WDM) are being considered as candidates for such high capacity optical transmission systems. For example, with regard to TDM techniques, a significant amount of worldwide research is being performed on 40 Gb/s systems.
Chromatic dispersion (group-velocity dispersion (GVD)) is one of the factors limiting the transmission distance in a 40 Gb/s system. Since dispersion tolerance is inversely proportional to the square of the bit rate, the dispersion tolerance, which is about 800 ps/nm at 10 Gb/s, is reduced by a factor of 16 to about 50 ps/nm at 40 Gb/s.
For example, in measured experiments, an optical time-division multiplexed (OTDM) signal with a signal light wavelength of 1.55 .mu.m (where transmission loss in silica fiber is the lowest) was transmitted over a distance of 50 km through a single-mode fiber (SMF). The SMF had a zero dispersion wavelength of 1.3 .mu.m. This type of SMF is the type of fiber most widely installed around the world. The input signal light power was +3 dBm, and the bit rate was 40 Gb/s. Dispersion compensation was performed using a dispersion-compensating fiber (DCF). The width of the dispersion compensation value range allowed in order to hold the power penalty (degradation of optical signal reception sensitivity through transmission) to within 1 dB (dispersion compensation tolerance) was 30 ps/nm. This value translates to length of 2 km or less of an SMF with a chromatic dispersion value of 18.6 ps/nm/km.
Furthermore, in a land system, repeater spacing is not uniform. Thus, very precise dispersion compensation must be performed for each repeater section.
On the other hand, dispersion in a transmission line changes with time due to changes, for example, in temperature. For example, in the case of an SMF 50 km transmission, when the temperature changes between -50 to 100.degree. C., the amount of change of the transmission line dispersion is estimated to be as follows:
(Temperature dependence of zero dispersion wavelength of transmission line).times.(Temperature change).times.(Dispersion slope).times.(Transmission distance)=0.03 nm/.degree. C..times.150.degree. C..times.0.07 ps/nm .sup.2 /km.times.50 km=16 ps/nm.
This value is greater than one half of the dispersion tolerance of 30 ps/nm and cannot be overlooked when designing the system.
In the above-described measured experiments, when the amount of dispersion compensation was optimized at -50.degree. C. at the start of system operation, if the temperature subsequently rose to 100.degree. C. during system operation, the criterion of a 1 dB penalty could not be satisfied (worst case condition).
Further, depending on the characteristic and construction of a dispersion compensator, the amount of dispersion compensation can only be set discretely, sometimes leaving no alternative but to set the dispersion compensation amount to a value slightly displaced from an optimum value at the start of system operation. In that case, there arises a possibility that the criterion of 1 dB penalty may not be satisfied even when the temperature change is smaller than 150.degree. C.